Tissues were collected from ailing monkeys and submitted to two separate laboratories for analysis. Polychlorinated biphenyls (PCB's) were suspected based on clinical and pathologic observations. This was confirmed using gas chromatography and mass spectrographic techniques. The source of PCB's was a concrete sealer used on the outdoor runs. The surface of the runs has been coated with polyurethane, and the problems related to PCB toxicosis have abated. The overall health of the colony appears to have greatly improved.